Yes, you are :)

There are a couple of optimisations you can make here. Firstly, you have may constructs like:

for ('something' .. 'something else') {
my $var = $_;
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where you can use:

for my $var ('something' .. 'something else') {
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Secondly, and more importantly, at each level you initialise the elements to empty lists, with code like:

$bucket{$x}{$y} = ();
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But on the next level down ,you overwrite it with an reference to a hash. The initial assignment is unnecessary.

In fact, you can simply the whole thing to:

for my $month ('02'..'03') {
  for my $day ('00' .. $mons{$month}) {
    for my $hour ('00' .. '24') {
      for my $minute ('00' .. '59') {
        $bucket{$month}{$day}{$hour}{$minute} = [0,0];
      }
    }
  }
}
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"Perl makes the fun jobs fun
and the boring jobs bearable" - me

If performance matters then temporaries may be your friend. For instance the following should run several times faster than your code:

foreach my $month ('02'..'03') {
  my $m_bucket = $bucket{$month} = {};
  foreach my $day ('00' .. $mons{$month}) {
    my $d_bucket = $m_bucket->{$day} = {};
    foreach my $hour ('00'..'23') {
      my $h_bucket = $d_bucket->{$hour} = {};
      foreach my $minute ('00'..'59') {
        $h_bucket->{$minute} = [0,0];
      }
    }
  }
}
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(A useful trick I have put to good use in the past.)

doh!! :),
Plus I just realised that hours in the day don't go from 00 to 24,
well not unless you've had a few too many beers